As is known, Roots-type blowers are similar to gear pumps in that both employ toothed or lobed rotors meshingly disposed in transversely overlapping cylindrical chambers. Adjacent nonmeshing lobes of each rotor transfer volumes of inlet port fluid to the outlet port. When the lobes remesh, outlet port fluid is trapped in contracting spaces between the meshing lobes and compressed unless venting is provided. When the rotor lobes are straight, i.e., parallel to the rotor axis, outlet vents have been provided for returning a portion of the trapped fluid to the outlet port and inlet vents have been provided for returning the remainder of the trapped fluid to the inlet port. However, when helical lobes are employed, known outlet vents have not been provided since such outlet vents would provide a leak path from the outlet port to the inlet port via expanding spaces between the meshing lobes. Examples of gear pumps with outlet and inlet vents may be seen by reference to U.S. Pat. Nos. 3,113,524; 3,303,792; and 4,130,383, which are incorporated herein by reference. Examples of Roots-type blowers with helical lobes and inlet vents may be seen by reference to U.S. Pat. Nos. 4,556,373 and 4,569,646, which are incorporated herein by reference.